1. Field of the Invention
The present invention relates to a trim-tilt device for a marine propulsion device for an outboard motor, an inboard motor and the like.
2. Description of the Related Art
There is known in the art a conventional trim-tilt device for a marine propulsion device, which comprises a boat body and a cylinder device interposed between the boat body and the marine propulsion device tiltably supported on the boat body, and in which a hydraulic fluid is controlled for passage from a hydraulic fluid supply-discharge device to the cylinder device and for discharged from the cylinder device to the hydraulic fluid supply-discharge device so that the cylinder device is expanded and contracted to produce a trim action and a tilt action of the marine propulsion device. More specifically, the trim action is for obtaining the best travel state by tilting the marine propulsion device against the propulsive force within a predetermined trim region for trimming up and down so as to adjust the trim of the marine propulsion device. This action requires a relatively large propulsive force of the cylinder device. The tilt action tilts the marine propulsion device at anchor against its gravity within a predetermined tilt up and down region so as to raise the marine propulsion device from the surface of the water. In this case, a relatively small propulsive force of the cylinder device suffices.
There is in the prior art a simple trim-tilt device capable of conducting trim and tilt action using a single hydraulic cylinder as described in Japanese Patent Application Publication (JP-B) No.60-50635.
In the prior art of Japanese Patent Application Publication (JP-B) No.60-50635, although it is unnecessary to provide two cylinders, i.e., a trim cylinder and a tilt cylinder, it is necessary to provide, in a single cylinder, three pistons, i.e., a tilt piston, a free piston and a trim piston and therefore, the structure of the cylinder device is more complicated.
Further, in the marine propulsion device, in order to prevent an engine from being damaged by a propeller which rises higher than the surface of the water and rotates at a higher speed at the time of trim up action of the traveling marine propulsion device, it is necessary to stop the trim action, without fail, at a position before the propeller rises higher than the surface of the water.